Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers have been used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction. Pacing therapy may also be used to treat ventricular conduction disorders by pacing both ventricles in order to result in a more coordinated contraction, termed cardiac resynchronization therapy.
In pacemaker patients who are chronotropically incompetent (e.g., sinus node dysfunction), the heart rate is determined solely by the pacemaker in the absence of intrinsic cardiac activity. That heart rate is determined by the programmed escape intervals of the pacemaker which cause paces to be delivered to the atria and/or ventricles, depending upon the pacing mode, if no intrinsic beats occur before expiration of the escape intervals. Pacing the heart at a fixed rate as determined by the length of the programmed escape intervals, however, does not allow the heart rate to increase with increased metabolic demand. It is for this reason that rate-adaptive pacemakers have been developed which vary the programmed escape intervals in accordance with one or more physiological parameters related to metabolic demand such as obtained from an accelerometer or minute ventilation sensor. In chronotropically competent patients in need of ventricular pacing, on the other hand, atrial triggered pacing modes such as DDD or VDD are desirable because they allow the pacing to track the physiologically normal atrial rhythm, which causes cardiac output to be responsive to the metabolic needs of the body. For this latter group of patients, the pacemaker is normally programmed so that the atrial rate is overridden by an atrial or ventricular pace only if the atrial rate drops to a level considered unsafe.
In a chronotropically competent patient with cardiac disease, the change in heart rate in response to exercise is a useful indicator of the patient's cardiopulmonary functional status. A decrease in cardiopulmonary function, for example, often manifests as an increased heart rate for a given workload since the heart must beat faster to produce the needed cardiac output. An improvement in cardiopulmonary function, on the other hand, may be indicated by a decreased heart rate with the same workload. Evaluation of cardiopulmonary function in this manner is normally performed by exercise testing in a formal clinical setting. The present disclosure relates to a way of assessing a patient's cardiopulmonary function by utilizing the sensing capabilities of an implantable cardiac device.